As Alf correctly identified, it's a (frosty) fairlead.
Is the point of them to protect the paintwork? They certainly don't seem to protect the ropes.
Moving on and meeting up at Bourne End - Cookham to Bourne End, River Thames, 1½ miles 0 locks Mum brought the family along yesterday evening but Meg was having none of it looking out of the back ...
3 hours ago